Because of the differences in language and character use, interpreting written …

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Our awareness that some individuals have great difficulty parsing the written word, a condition now termed dyslexia, apparently dates back over 100 years. With the advent of modern biomedicine, however, we've come to appreciate it as a disorder of the central nervous system. A number of genes have been associated with a increased probability of developing dyslexia in some populations. Specific regions in the brains of dyslexics have also been found to have reduced volume and activity. A study that will appear in PNAS this week adds a new twist to our understanding of the disease—it turns out that dyslexia in readers of Chinese may be a very different thing from the disorder that strikes readers of alphabetic languages.

In retrospect, this shouldn't be a huge surprise. In contrast to the finite set of alphabetic characters, Chinese writing includes a huge variety of complex symbols, each representing a different word. Spoken Chinese is also tonal, meaning that different words can be represented by sounds that differ in tone and inflection, rather than simply the order of sounds. As such, reading Chinese could be expected to tax the visual system, and possibly engage the auditory system, a lot more than a nontonal, alphabetic language.

The new study extends some previous work in the area by showing that there are changes in both the structure and activity in a number of brain regions in Chinese dyslexics, and these differ from the areas affected in English speaking dyslexics. Basic scans using MRI, followed by volumetric analysis, revealed a number of brain regions that were somewhat smaller in Chinese dyslexics compared to control Chinese speakers, but the most significant difference is in the left middle frontal gyrus. This area has been associated with managing working memory, and is thought to be involved in recalling the syllable that corresponds to a given character. The volume of this region had a strong correlation with scores on a reading ability test, even in nondyslexic individuals, which emphasizes its importance.

The researchers also put the subjects through an fMRI test, in which they were asked to determine if two characters displayed on a screen rhymed. The left middle frontal gyrus again appeared as having a significant difference in activity in dyslexics as compared to the control subjects, but a number of other areas also displayed differences. The researchers suggest that these other differences in activity may reflect regions recruited to compensate for the low activity, but it seems equally likely that the other regions depend on left middle frontal gyrus activity in some way.

The work is an interesting demonstration of how something that appears to be a single mental phenomenon results from two completely different biological processes. The authors also point out that the differences could be the product of an interplay between culture and biology. Chinese students are taught to memorize characters in part by repeatedly reproducing them; a brain region involved in that appears to be physically very close to the one identified in this study.